Numerical simulation of microbubble-assisted ultrasound therapy for thermal ablation*

A fully compressible multiscale model for the simulation of microbubble assisted High-Intensity Focused Ultrasound (HIFU) is presented. HIFU is a non-invasive therapy where high-intensity ultrasound waves are focused onto a target tissue to cause thermal ablation as a result of localized energy deposition. The non-linear ultrasonic field is modeled using compressible Navier-Stokes equations on a fixed grid, while the microbubbles are tracked as discrete singularities in a Lagrangian fashion. These two models are coupled to each other such that both the acoustic field and the bubbles influence each other. The energy absorbed by the medium locally due to the focused ultrasound and bubble dynamics is then used to compute the temperature rise in the focal and surrounding regions by solving a bio-heat transfer equation over the entire insonation time period. We first demonstrate the HIFU simulation without microbubbles and characterize the pressure and temperature fields by validating against available experiments. Experimental validation in the presence of microbubbles is then carried out to demonstrate the accuracy of the model. We then study the effect of microbubbles in the focal region on altering the energy absorption by the tissue.